One important criteria for safely operating a motor vehicle is the maintenance of proper pressure within tires of the vehicle. If tire pressure is not maintained, fuel economy is adversely affected and rates of tire wear increase. Additionally, handling of the vehicle can be adversely affected and the potential for abrupt failure of the tire is increased. Hence, it is advisable to monitor the pressure within vehicle tires on a regular basis.
One of the difficulties encountered in such regular tire pressure monitoring is that a significant amount of time is associated with monitoring tire pressure. In particular, a safety cap must be removed from the valve stem (and not misplaced). The gauge must be placed upon the valve stem of the tire. Care must be taken to make sure that a significant amount of air is not lost from the tire during this gauge installation process. The pressure can then be read from the gauge. If the tire pressure is adequate, the procedure is completed for that tire and the next tire can be checked. On a vehicle with multiple tires, this basic routine can be significantly time consuming, decreasing the likelihood that tire pressure monitoring will occur on a sufficiently frequent basis.
Compounding this difficulty is the inconvenience encountered when tire pressure is low and air needs to be added to the tire. First the gauge is removed. Then the vehicle is brought into proximity with a source of compressed air. Then compressed air is added to the tire. Unless an accurate gauge is associated with the source of compressed air, the source of compressed air must be removed after a relatively short period of time and the pressure gauge reinserted on the valve stem to take another reading. Typically, a half a dozen or so filling steps and measuring steps are involved before the tire pressure has been brought to the proper level.
It is known in the prior art to provide tire pressure gauges which are mountable upon a valve stem and which are sufficiently small and lightweight that they can remain mounted to the tire during operation of the tire. While this solves a portion of the problem, the difficulty associated with adding air to the tire is not alleviated. In at least two prior art patents, tire pressure gauges are taught which further allow for a source of compressed air to pass into the valve stem of the tire while the gauge remains upon the valve stem. In particular, U.S. Pat. No. 5,377,539 to LaSalle and U.S. Pat. No. 4,924,697 to Hunt (each incorporated herein by reference in their entirety) disclose tire pressure gauges which can mount on a valve stem of a tire and which also include a port through which compressed air can be added without removing the tire pressure gauge.
The tire pressure gauges taught by LaSalle and Hunt are not entirely satisfactory. In particular, in the case of LaSalle the fill port passes through a center of a face which is read to determine the pressure of the tire. A Bourdon tube supporting an indicator is wrapped around this fill tube. To accommodate this fill port passing through a center of the Bourdon tube, a customized Bourdon tube is required which significantly complicates the manufacture of the gauge taught by LaSalle. Also, reading the pressure indicated by the Bourdon tube is made more difficult by the presence of a valve core in a middle of a face of the gauge.
In the case of the device taught by Hunt, the fill port is provided separate from the face of the gauge. However, the spiraling Bourdon tube is not fully supported for the extreme operating environment experienced when the Hunt gauge remains mounted upon the tire, and as the tire is rotating at high speed, such as when the vehicle is in operation. These rotational forces impart a high degree of centrifugal force upon the Bourdon tube, tending to exceed an elastic limit of the materials and/or induce creep into the materials from which the Bourdon tube is formed, and potentially bringing the Bourdon tube out of proper calibration. The Bourdon tube is particularly susceptible to damage in that it must purposefully be made of sufficiently thin material to allow it to flex in response to pressure changes. Thus, over time the gauge taught by Hunt has a potential to become less accurate.
In attempting to compensate for this tendency, added complexity, added weight and other negative attributes can become necessary to implement the invention taught by Hunt. Accordingly, a need exists for a tire pressure gauge which can remain upon a tire and which includes an appropriate support surface to help to keep the Bourdon tube from being flexed out of an original position when the gauge is experiencing centrifugal forces and other forces associated with rotating along with a tire upon which the Bourdon tube and associated gauge are mounted.